docs: def _load_cache(self, filename):
          """Load the cached page references from `<filename>.ptc`."""
          try:
              with open(filename + self.CACHE_EXTENSION, 'rb') as file:
                  prev_number_of_pages, prev_page_references = pickle.load(file)
          except (IOError, TypeError):
              prev_number_of_pages, prev_page_references = {}, {}
          return prev_number_of_pages, prev_page_references